Shiny things have hard white highlights. If you observe any shiny object, you will easily see how there are sharp transitions between color and bright white at specific spots. The position and shape of the highlight is very important and depends on the position of the light and the shape of the object (hence needs to be consistent with the overall lighting of your scene), but you can wing the shape as long as the highlights are in the right position.
So you can use masking fluid on the highlights and keep them masked until the very end, or use white gouache or any other opaque white medium at the end.

An interesting thing to look at might be the paint work on cars. Very shiny, but don’t use the white highlight thing. A few months ago I started having to paint and colour-draw cars and someone pointed out to me. The paint doesn’t so much reflect like chrome, but it’s almost an interaction of shadow reflection with lighter and darker areas to create form.
As a caveat, the darker the paintwork, the more colour reflective the surface. So bright colours reflect more like a mirror, but the days are still more sillouhettes.

There's an iridescent medium that golden makes, you can mix it into your watercolor or just brush it on top and it makes everything nice and shiny. There are some daniel smith paints that are iridescent too but the medium gives you more range imo

All good suggestions. Note: If an object is underwater, it will not have hard highlights and they wont be white. Everything underwater takes on the hue of water and is a little blurry in detail. Wet things coming out of the water get hard highlights. That said, here's a pretty good example of a shiny surface object in watercolor. https://www.youtube.com/watch?v=dcmfRooP6dg